If I have a util class with static methods that will call Hibernate functions to accomplish basic data access. I am wondering if making the method synchronized
is the right approach to ensure thread-safety.
I want this to prevent access of info to the same DB instance. However, I'm now sure if the following code are preventing getObjectById
being called for all Classes when it is called by a particular class.
public class Utils {
public static synchronized Object getObjectById (Class objclass, Long id) {
// call hibernate class
Session session = new Configuration().configure().buildSessionFactory().openSession();
Object obj = session.load(objclass, id);
session.close();
return obj;
}
// other static methods
}
To address the question more generally...
Keep in mind that using synchronized on methods is really just shorthand (assume class is SomeClass):
synchronized static void foo() {
...
}
is the same as
static void foo() {
synchronized(SomeClass.class) {
...
}
}
and
synchronized void foo() {
...
}
is the same as
void foo() {
synchronized(this) {
...
}
}
You can use any object as the lock. If you want to lock subsets of static methods, you can
class SomeClass {
private static final Object LOCK_1 = new Object() {};
private static final Object LOCK_2 = new Object() {};
static void foo() {
synchronized(LOCK_1) {...}
}
static void fee() {
synchronized(LOCK_1) {...}
}
static void fie() {
synchronized(LOCK_2) {...}
}
static void fo() {
synchronized(LOCK_2) {...}
}
}
(for non-static methods, you would want to make the locks be non-static fields)
By using synchronized on a static method lock you will synchronize the class methods and attributes ( as opposed to instance methods and attributes )
So your assumption is correct.
I am wondering if making the method synchronized is the right approach to ensure thread-safety.
Not really. You should let your RDBMS do that work instead. They are good at this kind of stuff.
The only thing you will get by synchronizing the access to the database is to make your application terribly slow. Further more, in the code you posted you're building a Session Factory each time, that way, your application will spend more time accessing the DB than performing the actual job.
Imagine the following scenario:
Client A and B attempt to insert different information into record X of table T.
With your approach the only thing you're getting is to make sure one is called after the other, when this would happen anyway in the DB, because the RDBMS will prevent them from inserting half information from A and half from B at the same time. The result will be the same but only 5 times ( or more ) slower.
Probably it could be better to take a look at the "Transactions and Concurrency" chapter in the Hibernate documentation. Most of the times the problems you're trying to solve, have been solved already and a much better way.
For a class (static) method, the monitor associated with the Class object for the method's class is used. For an instance method, the monitor associated with this (the object for which the method was invoked) is used.
Thus if one thread enters a static method, the same object returned by Object#getClass is locked. Other threads can still access instance methods.
Class
object, instantiated by one of the virtual machines classloaders. Like all objects, this object too has a Monitor
associated with it. And this monitor is what is being locked.
Static methods use the class as the object for locking, which is Utils.class for your example. So yes, it is OK.
static synchronized
means holding lock on the the class's Class
object where as synchronized
means holding lock on that class's object itself. That means, if you are accessing a non-static synchronized method in a thread (of execution) you still can access a static synchronized method using another thread.
So, accessing two same kind of methods(either two static or two non-static methods) at any point of time by more than a thread is not possible.
Why do you want to enforce that only a single thread can access the DB at any one time?
It is the job of the database driver to implement any necessary locking, assuming a Connection
is only used by one thread at a time!
Most likely, your database is perfectly capable of handling multiple, parallel access
If it is something to do with the data in your database, why not utilize database isolation locking to achieve?
To answer your question, yes it does: your synchronized
method cannot be executed by more than one thread at a time.
How the synchronized Java keyword works
When you add the synchronized
keyword to a static method, the method can only be called by a single thread at a time.
In your case, every method call will:
create a new SessionFactory
create a new Session
fetch the entity
return the entity back to the caller
However, these were your requirements:
I want this to prevent access to info to the same DB instance.
preventing getObjectById being called for all classes when it is called by a particular class
So, even if the getObjectById
method is thread-safe, the implementation is wrong.
SessionFactory best practices
The SessionFactory
is thread-safe, and it's a very expensive object to create as it needs to parse the entity classes and build the internal entity metamodel representation.
So, you shouldn't create the SessionFactory
on every getObjectById
method call.
Instead, you should create a singleton instance for it.
private static final SessionFactory sessionFactory = new Configuration()
.configure()
.buildSessionFactory();
The Session should always be closed
You didn't close the Session
in a finally
block, and this can leak database resources if an exception is thrown when loading the entity.
According to the Session.load
method JavaDoc might throw a HibernateException
if the entity cannot be found in the database.
You should not use this method to determine if an instance exists (use get() instead). Use this only to retrieve an instance that you assume exists, where non-existence would be an actual error.
That's why you need to use a finally
block to close the Session
, like this:
public static synchronized Object getObjectById (Class objclass, Long id) {
Session session = null;
try {
session = sessionFactory.openSession();
return session.load(objclass, id);
} finally {
if(session != null) {
session.close();
}
}
}
Preventing multi-thread access
In your case, you wanted to make sure only one thread gets access to that particular entity.
But the synchronized
keyword only prevents two threads from calling the getObjectById
concurrently. If the two threads call this method one after the other, you will still have two threads using this entity.
So, if you want to lock a given database object so no other thread can modify it, then you need to use database locks.
The synchronized
keyword only works in a single JVM. If you have multiple web nodes, this will not prevent multi-thread access across multiple JVMs.
What you need to do is use LockModeType.PESSIMISTIC_READ
or LockModeType.PESSIMISTIC_WRITE
while applying the changes to the DB, like this:
Session session = null;
EntityTransaction tx = null;
try {
session = sessionFactory.openSession();
tx = session.getTransaction();
tx.begin();
Post post = session.find(
Post.class,
id,
LockModeType.LockModeType.PESSIMISTIC_READ
);
post.setTitle("High-Performance Java Perisstence");
tx.commit();
} catch(Exception e) {
LOGGER.error("Post entity could not be changed", e);
if(tx != null) {
tx.rollback();
}
} finally {
if(session != null) {
session.close();
}
}
So, this is what I did:
I created a new EntityTransaction and started a new database transaction
I loaded the Post entity while holding a lock on the associated database record
I changed the Post entity and committed the transaction
In the case of an Exception being thrown, I rolled back the transaction
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